If a heat pump system, such as for example an air conditioning system, is configured to operate as a heater, the corresponding exchanger or radiator installed in the external environment will operate as an evaporator and, for this reason, the temperature of its surface is fairly low.
When the external air is cold as well, typically during winter, with varying percentages of humidity, frost or ice will form on the surface of the external evaporator, causing a consequent reduction in the efficiency of the heat exchange, mainly owing to the insulating capacity of the ice and to the decrease in the spacing between the fins of the external evaporator.
Substantially, if the external radiator or exchanger operating as an evaporator is not periodically defrosted, the operation, and also the efficacy and efficiency, of the heat pump system will be negatively and considerably affected.
Usually, when the layer of frost or ice on the external evaporator is excessive, the power of the heat pump system will be reduced, the evaporation pressure of the cooling fluid will be modified, and malfunctions can arise, such as for example:                a possible return of coolant gas in the liquid phase during suction by the compressor, causing damage to or the total breakage thereof,        constant and sudden triggering of the deicing system, causing a waste of energy;        a very low output of warm air from the internal exchanger operating as a condenser;        a drastic lowering of the performance coefficient (up to 30%) from the performance specifications given by the maker.        
The aim of the deicing cycle, also known as the defrosting cycle, is therefore to melt such frost or ice that has formed on the surface of the external evaporator; it can be carried out with different methods, according to the type of system and the different requirements.
The method of deicing that is used the most, in particular in the field of air conditioning, takes advantage of the possibility to combine both the heating function and the cooling function in a single heat pump, thus making it possible to proceed with the periodic deicing of the external evaporator by way of a cycle inversion, which makes it possible to make the high-temperature cooling fluid originating from the compressor, typically in the form of a gas, pass into the external evaporator to be deiced.
In conventional heat pump systems, such as for example conventional air conditioning systems, in order to melt this layer of ice, a reversible valve temporarily inverts the cycle of the cooling fluid, so as to change the direction of the flow of heat; in this way the roles are also inverted of the external radiator, which passes from acting as an evaporator to acting as a condenser, and of the internal radiator, which passes from acting as a condenser to acting as an evaporator.
Therefore, in a deicing cycle, the cooling fluid evaporates in the internal radiator and condenses in the external radiator, the internal and external ventilations stop, so as to reduce the heat energy necessary for the deicing, and the compressor compresses gas at high temperature in the external radiator, thus making it possible to melt the ice that has formed.
Usually, conventional heat pump systems have two or three deicing cycles per hour, which are executed at an external air temperature of +4÷5° C. and as a function of the humidity present.
Obviously, while the heat pump is in this deicing step, the internal radiator cools the air that is intended for example for the rooms of a building to be heated, and therefore there is a necessity to heat the air before putting it into circulation (this is known as preheating).
One of the biggest problems relates to the correct adjustment of the frequency of the deicing cycles. In fact, infrequent deicing cycles lead to the formation of ice very often on the surface of the external evaporator, worsening the heat exchange efficiency; while over-frequent deicing cycles lead to the introduction of cold air into the air conditioning system, with negative effects on the wellbeing of the end users, and energy waste, for example owing to frequent cooling fluid cycle inversions or to repeated preheating operations.
The adjustment of the duration of the deicing cycles is also strategic to the complete melting of the ice. In fact, if the deicing step is too short, not all of the frost or ice that is present on the external evaporator will be melted, and the remaining part tends to solidify more thickly and compactly when the deicing step ends and operation returns to the heating step.